1. Field of the Invention
This invention relates to an automatic transmission, and more particularly to an apparatus for controlling line pressure of an electronically controlled automatic transmission.
2. Description of the Prior Art
An electronically controlled automatic transmission of the prior art generally includes a line pressure regulating valve for regulating the pressure of operating fluid from an engine-driven oil pump at a predetermined pressure level, and a vacuum throttle valve coacting with a vacuum diaphragm to which a negative pressure corresponding to the degree of throttle opening representing engine load is applied, so that the vacuum throttle valve responds to the aforesaid negative pressure for producing a throttle pressure representing the engine load. The throttle pressure is led to the aforesaid line pressure regulating valve, so as to regulate the line pressure at a level corresponding to the engine load. The line pressure thus regulated is selectively applied to those clutches, brakes, or the like friction elements which are necessary for changing power transmission paths of planetary gear sets or necessary for producing the gear tatio being selected, whereby the friction elements are selectively actuated (in other words, applied or released) for effecting the desired automatic speed change.
In the electronically controlled automatic transmission, it is contemplated to add an automatic engine brake selecting circuit, which circuit detects conditions necessitating the engine brake and generates an engine brake instruction, for automatically effecting shift-down (i.e., selection of a larger speed reduction ratio) and automatically achieving more effective engine brake action. The conditions necessitating the engine brake occur during drive (D) range operation of the automatic transmission at the highest gear ratio thereof, for instance, if a car is accelerated at a rate exceeding a certain value even after the driver of the car releases his foot from an accelerator pedal and depresses a brake pedal.
However, when the engine brake is applied in the aforesaid manner, the driver releases his foot from the accelerator pedal, and the negative pressure acting on the vacuum diaphragm is increased for reducing the throttle pressure and minimizing the line pressure. Accordingly, the torque transmitting capacity of those friction elements which correspond to the low gear ratio to be actuated by the minimized line pressure is also minimized. On the other hand, to carry out the engine brake effectively, the aforesaid friction elements are required to have a large torque transmitting capacity, so that shortage of torque transmitting capacity is inevitably caused. Consequently, the automatic engine brake circuit of the prior art has difficulties in that the friction elements to be actuated during the engine brake tend to slip due to the fact that they can use only small dynamical friction coefficients in the beginning, and hence reliable engine brake action cannot be achieved. Besides, the engine brake selecting circuit has drawbacks in that the aforesaid slip tends to accelerate wear and tear of the friction elements and to shorten their service lives.